1. The Technical Field
The present invention is directed generally to the field of fiber optics. More particularly, the present invention is directed to an apparatus and method for effecting an optical U-turn in an optical fiber in a small space.
2. The Prior Art
In highly-integrated fiber optic assemblies, there often is a need to turn a fiber through 180xc2x0 in a very small space. Simply bending the fiber tightly is usually not satisfactory because tight bends in optical fibers cause high bending losses which reduce the fiber""s efficiency and high bending stresses which aggravate the risk of fiber failure.
One approach to effecting an optical U-turn in a small space includes etching and/or heating and drawing the optical fiber to reduce its diameter to a few microns. The reduced diameter section is then formed into a tight 180xc2x0 bend. The resulting device could be used as-is, but would be very fragile and prone to breakage. However, the reduced diameter section can be protected by encapsulating it in a suitable encapsulant. A low refractive index encapsulant can be used to confine the mode field in the reduced diameter section, such that the resulting U-turn achieves low optical loss. In principle, a U-turn device constructed in this way might be only slightly more than two fiber diameters in width. U.S. Pat. No. 5,138,676 to Stowe, et al. entitled xe2x80x9cMiniature Fiberoptic Bend Device and Methodxe2x80x9d describes this approach.
The foregoing technique has several drawbacks. For example, the tapered transition region between the original fiber diameter and the reduced diameter section must be precisely controlled to minimize light loss. This requires specialized equipment and control algorithms. Also, the reduced diameter section is extremely fragile. Thus, successfully fabricating and packaging such a U-turn device would be very challenging. Further, it is not clear that such devices would be reliable over the long term. For example, temperature fluctuations could produce severe stresses in the reduced-diameter section if the thermal properties of the encapsulant were not closely matched to those of the glass fiber.
Another way to effect an optical U-turn in a fiber is to cleave the fiber and use discrete mirrors or a prism, along with the required alignment optics, to turn the light. FIG. 1 illustrates one such type of device. The device includes a first collimator 12 connected to a first optical fiber 10 and a second collimator 16 similarly connected to a second optical fiber 14. First and second collimators 12 and 14 are each connected to a discrete, miniature 90xc2x0 prism 18. Typically, the 90xc2x0 surfaces of prism 18 are surrounded by a low index medium, such as air, and the light is turned by total internal reflection. While this simple technique overcomes many of the shortcomings of the bend-type device discussed above, achieving the alignment precision required for low loss with single mode fiber is difficult using discrete components, and the resulting size of the U-turn assembly typically would be much larger than a few fiber diameters.
It would be desirable to provide a method of fabricating a prism-type optical fiber U-turn device which overcomes the difficulties in achieving the required alignment precision. It also would be desirable to provide a prism-type optical U-turn device using an integral prism and a method of making such a device.
The present invention provides a method and apparatus for effecting a xe2x80x9cU-turnxe2x80x9d in an optical fiber with low loss in a space potentially only two fiber diameters wide. Such a device includes a prism, a first optical fiber which admits light to the prism (an input fiber), and a second optical fiber which carries light from the prism (an output fiber). Some embodiments further include a substrate for maintaining the necessary optical alignments between the prism and the first and second, or input and output, optical fibers.
The input and output fibers preferably are made of single mode optical fiber. Alternatively, they can be made of multimode fiber. In a preferred embodiment, a collimator is connected to the end of each of the input and output fibers, between the fibers and the prism. These collomators preferably are made of a gradient index optical fiber. In an alternate embodiment, a collimator is connected to only one of the input and output fibers. In another alternate embodiment, a focusing lens is connected to only one of the input and output fibers via an intervening optical coupler, such as a section of coreless fiber or an optical gel appropriately formed for this purpose. When an optical gel is used, it should be index-matched to the cladding of the fiber(s) it is connected to. That is, the refractive index of the optical gel should be substantially the same as the refractive index of the cladding of the fiber it is connected to.
The prism can be a discrete, commercially available component, or it can be custom fabricated for this application. In some embodiments, the prism can be custom molded from an index-matched adhesive or optical gel as part of the assembly process. In other embodiments, the prism can be machined or otherwise formed from a section of coreless optical fiber connected to the end of the input and output fibers.
The substrate, where used, preferably is machined from silicon using a deep reactive ion etching process or from nickel using a LIGA process. Alternatively, the substrate can be precision molded from a suitable plastic or other material. The substrate preferably includes one or more channels for locating and/or securing the input and output fibers (and/or the focusing devices connected thereto). In embodiments using a discrete prism, the substrate preferably is further configured to locate and/or secure the prism. In embodiments using a molded prism, the substrate preferably is further configured to provide a mold for forming the molded prism in situ. Preferably, a prism thus formed is integral with the input and output fibers (and/or the focusing devices connected thereto).